A marked-up copy of this amendment is enclosed herewith.
The invention relates to a method for operating an electric arc furnace to build up and maintain a foamed slag at a predetermined height over the entire cross-sectional surface of the melting bath by supplying a slag builder, in particular carbon.
Since the seventies, freely radiating arcs in a foamy slag are usually driven in the electric arc furnaces. The refractory masonries and water-cooling systems of the furnace are in this manner shielded from the strong emission load. Moreover the electric performance of the furnaces is also evened out. The conditions for the energy transfer through the arc changes constantly in particular during the warming phase due to the processes which take place between the electrode tip, the slag and the bath. In the case of a short-circuit between electrode and bath above the scrap metal, as it can, for example, be caused by a scrap-metal cave-in and bath fluctuations, the then yet supplied energy converts to graphite.
Measurements have shown that the entire electric efficiency of the energy transfer is merely 36%. Whereas if the arc dips with half of its length or, however, completely into the foamed slag, then the radiation energy of the arc is also transferred at half of its amount or completely through heat conduction of the slag to the steel bath. The energy utilization increases then to 65 to 93%. An optimum energy feed input is achieved when a sufficient slag height is available and when the slag due to its specific conductivity can reproduce the arc resistance. The efficiency can hereby rise up to 100% through the additional reduction of the focal-spot losses.
A method for controlling the building of foamed slag in a rotary-current electric arc furnace is known from the EP-A1-0 692 544, in which the feeding of carbon for the building of foamed slag is specified in such a manner that both at least a partial enveloping of the arcs is realized and also an overdosed feeding of carbon is avoided. Here in order to control the throughput rate of carbon an automatic detection of a sound emission of the electric arc furnace is carried out. The amplitude of the sound is evaluated. When a specified sound level is exceeded, the throughput rate is increased, and when the specified sound level is less, the throughput rate is reduced. However, this arrangement is extremely unsatisfactory. The difficulty is now to adjust the slag height to an optimum.
The disadvantage of these conventional methods is that by using the lance manipulators in the area of the slag door, a considerable amount of space is needed, and moreover the lances must be replaced often. Due to the blowing in of the additives in the area of the slag door, the adjoining area preferably produces a high slag, whereas more or less slag exists in the remaining furnace areas. This has significant disadvantages. In particular, the slag forming takes place only in one area so that a sufficient slag is missing in the area opposite the slag door. This is mainly noticeable in electric arc furnaces which have an eccentric bay tapping feature. When in addition filter dust or other solids to be recycled are supplied through the lances, then the amount of the added solids is limited because the addition is concentrated at an area in the furnace area following the slag door. This easily results in formations of solid agglomerates which are difficult to melt. In addition, because of the uneven slag distribution, the know-how of the operating personnel is important most of all in deciding as to when and in what amount the additives must be blown into the electric furnace.
The basic purpose of the invention is to provide a method for operating an electric arc furnace to build up and maintain foamed slag, with which not only the energy consumption of the electric arc furnace is lowered as a whole, the melting process can be optimized, and also the top-to-top times can be shortened, and the wear of electrode material and refractory solids and the noise development can be reduced, but with which also a significantly quicker build-up of the slag with a uniform distribution in the surface and height of the electric arc furnace is achieved and also additional substances can be added in larger amounts than before.
This purpose is attained according to the invention by the amount of the slag builder fed into the electric arc furnace being regulated in dependency of the current absorbed by the electrodes and thus the absorption performance of the electric arc furnace regulated in such a manner that initially in order to build up the foamed slag, the maximum amount of slag builder is added until the current absorption by the electrodes is reduced, the amount of the added slag builder being subsequently reduced until the current absorption has reached a specified desired value, and the desired current absorption being maintained constant by subsequently reducing or increasing the added amount of slag builder.
Thus the basis of the method of the invention is that the absorption performance of the electrodes is reduced when the foamed slag has reached an optimum height. For this reason, initially at a specified absorption performance of the electric arc furnace, a maximum amount of foamed-slag builder, in particular carbon, is fed to the molten bath, namely, until the absorbed performance is reduced to a specified desired value. When the absorbed performance falls below this desired value, the addition of slag builders is reduced. When it increases again above the desired performance, the addition of slag builders is increased accordingly. This makes it possible to drive an optimum height of foamed slag, whereby the foamed slag is built up in a very short period of time. The slag builder is advantageously added through lances arranged in the wall of the furnace, whereby these lances extend through the wall of the furnace and are arranged approximately uniformly distributed over the periphery of the furnace. The lances are inclined with respect to the flow direction of the melt at an angle of approximately 30 to 80xc2x0. The inclination to the horizontal is 10 to 30 xc2x0.
The injection surface of the lances on the molten bath surface is chosen such that the injection surface lies in flow direction of the melt in each case in front of the electrode. This moreover assures that the slag is distributed uniformly over the entire surface of the bath. Also additives can moreover be supplied through the lances, whereby the amount of the supplied additives is significantly greater than is the case in common methods, in which these additives are added exclusively through the furnace door by means of lance manipulators. This results in an accumulation of additives in the area of the furnace door, which results in lump formation and thus in agglomerates which are very difficult to melt.